1. Technical Field
The present invention relates to an eddy current type brake system adapted for a vehicle and more particularly to an eddy current type brake system which produces eddy current in a rotor by use of permanent magnets.
2. Background Art
An eddy current type brake system is known as a vehicle retarder ensuring a stable continuous brake force in combination with a main brake (foot brake) and preventing burning of the main brake.
The eddy current type brake system generally includes a rotor mounted on a shaft, such as a propeller shaft, drivingly connected to wheels of a vehicle and a magnetic power source, such as an electro-magnet or a permanent magnet, mounted on a fixed portion, such as a frame of the vehicle, so as to be located near the rotor. The eddy current is produced in the rotor due to a relative velocity difference between the rotor (a rotary portion) and the magnetic power source (the stationary portion). The eddy current serves as the brake force against the rotation of the rotor, whereby the vehicle is decelerated.
When the eddy current flows in the rotor during the braking, the rotor becomes hot. The eddy current is considered as energy loss. Particularly, when the vehicle runs at a high speed, the rotor also rotates at a high speed and the time required for deceleration becomes long. Therefore, an amount of heat generated in the rotor becomes large and the temperature of the rotor raises rapidly.
The electrical resistance of the rotor becomes larger as the temperature of the rotor becomes high, which lowers the braking force of the rotor. Also, the heat of the rotor will deform the rotor. This deformation upsets the rotational balance of the rotor and/or lead to cracks in the rotor. Therefore, the longevity of the rotor is shortened.
The heat of the rotor is cooled by its own cooling capacity and it is advantageous for the rotor to have a large surface area in order to release the heat.
However, the space for the eddy current type brake system in the vehicle is limited. Thus, it is not possible to enlarge the surface area of the rotor by a large amount.